Methods and systems for resistance spot welding using direct current micro pulses

ABSTRACT

Methods and systems for resistance spot welding using direct current micro pulses are described. One described method comprises comprising forming a weld joint by applying a plurality of direct current micro pulses to at least two pieces of materials through a first electrode and a second electrode.

CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM TO PRIORITY

This application claims the benefit of U.S. provisional application Ser.No. 61/234,019 filed Aug. 14, 2009, entitled “Micro Pulsing ResistanceSpot and Seam Welding Method for Sheet Metal Joining,” the completedisclosure of which is incorporated herein by reference and to whichpriority is claimed.

FIELD OF THE INVENTION

The present invention relates to welding, and more particularly tomethods and systems for resistance spot welding using direct currentmicro pulses; including resistance spot welding, including a resistancespot welding method that can be used to weld the same or different sheetmaterials with extended electrode life, enhanced weld current range,large weld size (high welded joint strength) and the finest microstructure in the weld nugget. This method is particularly suitable for(but not limited to) joining sheet metals with different (heavy,oxidized) coatings, such as USIBOR, hot dip galvanized sheet steels etc.

BACKGROUND OF THE INVENTION

In a typical example of resistance spot welding, a pair of electrodesclamps two or more pieces of materials together by a predeterminedforce, and passes weld current between the tips of the electrodesthrough the pieces of materials. As the weld current flows through thepieces of materials, the resistance of the materials to the current flowcauses the materials to heat to their inherent melting point. Theresulting molten material solidifies under the predetermined clampingforce to form the welded joint, or nugget.

Conventional resistance spot welding processes used to weld two or morepieces of sheet materials together may apply alternating current (AC) ordirect current (DC). The operational current range is defined as theweld current values between the weld current for the designed minimumweld size (the minimum weld current) and the expulsion weld current (themaximum weld current). The weld current input may be one or more pulses.The time of each weld current pulse may range from one cycle per secondto sixty cycles or more per second.

The weld current range is defined as the difference between the lowerlimit (i.e. the minimum) weld current required to produce the minimumweld nugget size and the upper limit (i.e. the maximum) weld currentwhich causes molten metal splashing. Resistance spot welding (RSW)weldability tests have revealed that when DC weld current mode isemployed there is no stable weld current range for thin gauge (0.91 mm)USIBOR® 1500P and a very narrow weld current range for 1.52 mm USIBOR®1500P. RSW weldability tests have also shown that when AC weld currentis used there is a stable weld current range. Experimental resultsindicate that the deterioration rate of the electrode tip face for DC ismuch higher than that for AC. The use of higher weld force, longer weldtime and larger size electrodes may enlarge the weld current range forDC welding. However, the experimental results also discovered that theimprovement for electrode life is very limited from welding parameteroptimization.

Both low frequency direct current (DC) resistance welding equipment andmiddle frequency direct current (MFDC) resistance welding equipmentgenerate constant secondary DC current output for welding. The middlefrequency direct current (MFDC) resistance welding equipment utilizesfrequency pulses of 400 to 2,500 Hz instead of the frequency of basealternating current (50 or 60 Hz) to transform primary current intosecondary current. Thus, the size of MFDC welding equipment issignificantly reduced compared to AC and low frequency DC weldingequipment. The output welding current of MFDC resistance weldingequipment remains constant. Moreover, the MFDC welding equipment doesnot cause power supply line disturbances as is the case with lowfrequency DC and AC welding equipment.

MFDC resistance spot welding equipment is widely used in automotive,appliance and aircraft manufacturing industries because of its smallsize, light weight and controllability, and it is particularly suitablefor robotic applications. On the other hand, the size, weight, and/orcontrol of AC RSW equipment is not suitable for the same applications.Therefore, it would be advantageous to develop an innovative resistancespot welding method to obtain a robust resistance spot welding processwith enlarged weld current ranges, extended electrode life, finemicrostructure in the weld nugget, excellent welded joint strength, orany combination of these features.

SUMMARY OF THE INVENTION

Embodiments disclosed herein provide methods and systems for resistancespot welding using direct current micro pulses.

For example, one embodiment of methods and systems for resistance spotwelding using direct current micro pulses comprises a method comprisingthe steps of forming a weld joint by applying a plurality of directcurrent micro pulses to at least two pieces of materials through a firstelectrode and a second electrode. Another embodiment of methods andsystems for resistance spot welding using direct current micro pulsescomprises a system comprising a first electrode and a second electrodeconfigured to form a weld joint joining at least two pieces of materialstogether by applying a plurality of direct current micro pulses to theat least two pieces of materials.

Other embodiments and further details on various aspects of theinvention, including apparatus, systems, methods, kits, articles,assemblies, and the like which constitute part of the invention, willbecome more apparent upon reading the following detailed description ofthe exemplary embodiments and viewing the drawings. It is to beunderstood that the invention is not limited in its application to thedetails set forth in the following description, figures, and claims, butis capable of other embodiments and of being practiced or carried out invarious ways.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the invention arebetter understood when the following Detailed Description is read withreference to the accompanying drawings, wherein:

FIG. 1 is a flowchart of a first method for resistance spot weldingusing direct current micro pulses according to an embodiment of theinvention;

FIG. 2 is a flowchart of a second method for resistance spot weldingusing direct current micro pulses according to an embodiment of theinvention;

FIG. 3 is a block diagram of a system for resistance spot welding usingdirect current micro pulses according to an embodiment of the invention;

FIG. 4 is a graph showing an example of measured weld current wave formsfor a MPDC process, a conventional MFDC process, and a conventional ACprocess according to an embodiment of the invention;

FIG. 5 is a graph showing an example of measured weld current rangecomparisons of conventional MFDC process and a MPDC process according toan embodiment of the invention;

FIG. 6 is a graph showing an example of measured weld size and weldcurrent comparisons of a conventional MFDC process and a MPDC processaccording to an embodiment of the invention; and

FIGS. 7A and 7B are illustrations of the microstructures of the weldnuggets respectively formed by a conventional DC process and a MPDCprocess according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Reference will now be made in detail to exemplary embodiments andmethods of the invention as illustrated in the accompanying drawings, inwhich like reference characters designate like or corresponding partsthroughout the drawings. It should be noted, however, that the inventionin its broader aspects is not limited to the specific details,representative devices and methods, and illustrative examples shown anddescribed in connection with the exemplary embodiments and methods.

In one exemplary method for resistance spot welding using direct currentmicro pulses, a pair of electrodes fixes two pieces of materials (i.e.the workpiece) together by a predetermined weld or clamping force. Thepair of electrodes may comprise a copper based alloy, and fix the twopieces of material together by a designed weld force, such as about 5kN. A weld controller is in communication with the electrodes andconfigured to control one or more weld parameters, including the weldforce, weld current magnitude, weld duration, total number of welds, andoff-time duration.

The pair of electrodes applies a plurality of direct current micropulses to the at least two pieces of materials between the pair ofelectrodes. The plurality of direct current micro pulses can comprise aseries of short (e.g. one millisecond to ten milliseconds) directcurrent pulses separated by a series of short off-times. The magnitudeof each direct current micro pulse may be one to more than twentythousand amps (e.g. 5,000 amps). By using a sequence of direct currentmicro pulses, the two pieces of materials are locally melted, thusforming a weld joint which joins the two pieces of materials together.

Referring now to the drawings in which like numerals indicate likeelements through the several figures, FIG. 1 is a flowchart of a firstmethod for resistance spot welding using direct current micro pulsesaccording to an embodiment of the invention. The method begins with apair of electrodes applying a plurality of direct current micro pulsesto at least two pieces of material at 102. The materials may comprisevarious substances suitable for resistance spot welding, such as one ormore types of metal, such as steel. The gauge of the welding materialmay vary based on the design of the welded joint. For example, in oneembodiment, the at least two pieces of materials comprise two pieces of0.91 mm USIBOR® 1500P Steel.

Each direct current micro pulse may comprise a one to more than twentykiloamp (kA) pulse with a duration of 1 to 10 milliseconds (ms). Inother embodiments, a direct current micro pulse may last for about 1 to10 ms, and have a magnitude of 1-20 kA (i.e. 1000 amps-20000 amps). Eachdirect current micro pulse may be separated by a weld current off time.Each weld current off time may last for about 1 to 10 ms. During theweld current off time, no current or very low current is applied to theat least two pieces of materials.

In some embodiments, the weld current off times may be substantially thesame duration as the weld current on time (or micro pulse duration). Inother embodiments, the weld current off time is different than the weldcurrent on time.

By applying a plurality of direct current micro pulses, a weld jointjoining the at least two pieces of materials together is formed at 104.One or more weld joints, or weld nuggets, may be formed or created. Inone embodiment, a plurality of weld joints are created along a seambetween two materials.

FIG. 2 is a flowchart of a second method for resistance spot weldingusing direct current micro pulses according to an embodiment of theinvention. In step 202, at least two pieces of materials are fixedtogether. A pair of electrodes may fix the materials together at apredetermined weld force. The predetermined force may be about 1-10kilonewtons (kN). As one example, the electrodes may fix the pieces ofmaterial with a force of about 5 kN. In some embodiments, more than twopieces of material are fixed together.

Next, a direct current micro pulse is applied to the at least two piecesof materials at 204. The magnitude of each of the direct current micropulses may be between 1 kA and 20 kA. The duration of each of the directcurrent micro pulses may be between 1 ms and 10 ms.

After the direct current micro pulse is applied at 204, the weld currentis turned off during an off time at 206. During the off time 206, nocurrent, or a very low current, may be applied. The duration of the offtime may be between 1 ms and 10 ms.

Steps 204 and 206 may be repeated as necessary to form the weld joint.The weld strength is determined by the total number and duration of eachdirect current micro pulse 204 and off-time 206. During the method shownin FIG. 2, the direct current micro pulses 204 heat the localizedmaterial between the two electrodes to melting temperature, thus formingthe welded joint, or weld nugget.

FIG. 3 is a block diagram of a system for resistance spot welding usingdirect current micro pulses according to an embodiment of the invention.As shown in FIG. 3, the device comprises a first electrode 302 and asecond electrode 304. The first electrode 302 and the second electrode304 are both shown in communication with a weld controller 310.

The first electrode 302 and the second electrode 304 clamp, or holdtogether, a first piece of material 306 and a second piece of material308. The electrodes 302, 304 hold the materials 306, 308 together by apredetermined force. The first electrode 302 and a second electrode 304are configured to apply an electric pulse, such as a direct currentmicro pulse, to the workpiece.

The weld controller 310 may control various weld parameters of thewelding process. The weld parameters comprise the weld force, thewelding frequency (i.e. the duration of the weld pulses and the offtimes), total welding time (i.e. the total number of the weld pulses),and the welding current. The weld controller 310 may manipulate thevarious weld parameters based at least in part on specificcharacteristics of a weld project. For example, the weld controller 310may control the one or more weld parameters based at least in part onone of a joint design of a weld joint joining the at least two pieces ofmaterial together, a gauge of at least one of the at least two pieces ofmaterials, a coating of at least one of the at least two pieces ofmaterials, a material chemical composition of at least one of the atleast two pieces of materials, one or more mechanical properties of atleast one of the at least two pieces of materials, a size of the pair ofelectrodes, or a magnitude of the weld force.

In some embodiments, commercially available MFDC resistance spot weldingcontrollers may be used as the welding controller 310. As one example,the welding controller 310 may comprise the 3000 Series Welding Controlof Welding Technology Corporation.

FIG. 4 is a graph showing an example of measured weld current wave formsfor a MPDC process, a conventional MFDC process, and a conventional ACprocess according to an embodiment of the invention. The y axis, asshown on the left, represents current, measured in thousands of amps(kA). The x axis represents weld time, measured in milliseconds (ms).According to the embodiment shown in FIG. 4, waveforms, or measurementsof current over time, are shown for micro pulse direct current 402,direct current of MFDC welding equipment 404, and alternating current406.

As shown in FIG. 4, one conventional method for resistance spot weldingapplies a steady direct current 404 to the workpiece. The steady directcurrent 404 may be about 5 kA (this current varies from 1 to more than20 kA based on joined materials and gauge), and last for the duration ofthe welding process. Another conventional method for resistance spotwelding applies an alternating current 406 to the workpiece. As shown inFIG. 4, the alternating current 406 periodically alternates betweenabout negative seven (−7) kA and positive seven (+7) kA with a period ofabout 15 ms (the current varies from 1 to more than 20 kA based onjoined materials and gauge).

In contrast to conventional resistance spot welding methods, resistancespot welding using direct current micro pulses 402 uses a sequence ofshort direct current pulses, or micro pulses. Such short direct currentpulses (i.e. micro pulses) contrast with conventional current pulsingcycling, which typically involve pulsing spike widths in excess of 40ms. As shown in FIG. 4, each micro pulse lasts about 4 ms, and isfollowed by an off time lasting approximately 4 ms. The duration of thepulse current and the subsequent off time may only last a fewmilliseconds, for example about 1 to 10 ms. In one embodiment, thedirect current micro pulse and off time each last 1 ms. In otherembodiments, the direct current micro pulse and off time each last 2 ms,3 ms, 3.5 ms, 4 ms, 4.5 ms, 5 ms, or 10 ms.

The off time between direct current micro pulses may be of comparablelength, e.g., about 1 to 10 ms. In some embodiments, the length of thedirect current micro pulse is substantially equal to the length of theoff time. In other embodiments, the length of the direct current micropulse is different than the length of the off time. As one example, aweld having 100 direct current micro pulses may have a total weld timeof about 800 ms, with each direct current micro pulse lasting 5 ms, andeach off time cycle lasting 3 ms. The plurality of direct current micropulses may number anywhere from 3 or more, for example, 5, 10, 50, 80,100, or more micro pulses.

The magnitude of the weld current for each micro pulse, the weld currenton and off times, and the total number of micro pulses (or the totalweld time) may be adjusted from the weld current controller of theresistance spot welder based on the welded joint design. The magnitudeof the direct current micro pulse may be based at least in part on theproperties of the materials and gauge to be welded together. Forexample, the magnitude of the direct current micro pulse may be basedon: the gauge of the workpiece, a coating of the workpiece, materialchemical composition of the workpiece, mechanical properties of theworkpiece, size of the electrodes, applied weld force, and the totalweld times.

Micro pulse resistance spot welding offers several advantages overconventional spot welding processes. One advantage of micro pulseresistance spot welding is a longer useful lifetime of the electron tip.In conventional spot welding processes, electrodes may degrade, and havea useful life of less than 200 welds. However, using micro pulseresistance spot welding, electrode life may be extended to 500 or morewelds.

FIG. 5 is a graph showing an example of measured weld current rangecomparisons of conventional MFDC process and a MPDC process according toan embodiment of the invention. A conventional middle frequency directcurrent resistance spot welding process is represented by DC curves 502,504. A micro pulse direct current spot welding process according to oneembodiment of the invention is represented by MPDC curves 506, 508. Bothconventional and micro pulse direct current welds were measured during aweld process using 0.91 mm USIBOR® 1500P. During the weld process, apair of dome electrodes, each with a 6.0 mm diameter, clamped the 0.91mm USIBOR® 1500P with a force of 5.0 kN. The weld time was 20 cycles.

A first pair of curves, DC imprint diameter 502 and MPDC imprintdiameter 506, illustrates the diameter of an electrode tip imprintduring a tested welding process. For these imprint diameter curves, they axis on the left of the graph represents the diameter of the imprintin millimeters (mm), and the x axis represents number of welds. A secondpair of curves, DC current 504 and MPDC current 508, illustrates thecurrent range measured during a tested welding process. For thesecurrent range curves, the y axis on the right of the graph representsthe current in kA, and the x axis represents number of welds.

According to the MPDC imprint diameter 506, after 500 welds, the micropulse direct current welds maintained a relatively stable electron tipimprint size (about 4.5 mm compared to the original tip face diameter of5.0 mm) with a current range between about 2.11 kA and about 1.72 kA. Onthe other hand, as shown by the direct current imprint diameter 502, theelectrode tip imprint dropped below 4.0 mm after only 200 welds in aconventional process. Meanwhile, the weld current range of the directcurrent process is only 0.29 kA. Thus, the MPDC process provides alonger useful electrode lifetime and excellent weld current ranges ascompared to other conventional resistance spot welding processes.

Another advantage of micro pulse resistance welding is a larger weldcurrent range. FIG. 6 is a graph showing weld size and weld currentcomparisons of a conventional MFDC process and a MPDC process accordingto an embodiment of the invention. A conventional direct currentresistance spot welding process is represented by DC curve 604. A micropulse direct current spot welding process according to one embodiment ofthe invention is represented by MPDC curve 602. The y axis, shown on theleft, represents a weld size (i.e. size of a weld nugget or weld join)in millimeters (mm). The x-axis represents weld current in kA. Bothconventional and micro pulse direct current welds were measured during aweld process using 0.91 mm USIBOR® 1500P. During the weld process, apair of dome electrodes, each with a 5.0 mm diameter, clamped the 0.91mm USIBOR® 1500P with a force of 3.0 kN. The weld time was 14 cycles.

As shown in FIG. 6, a conventional direct current resistance spotwelding process 604 has a relatively small weld current range (fromabout 4.6 kA to 5.75 kA) for a reliable weld size of +4 mm is. On theother hand, the weld current range for resistance spot welding usingdirect current micro pulses is almost double, between about 4.75 kA and6.75 kA.

Methods and systems for resistance spot welding using direct currentmicro pulses may also produce larger weld joint sizes than conventionalresistance spot welding methods because of the wider weld current range.Compared to conventional welding processes such as MFDC, the MPDC weldcurrent ranger is much wider. Therefore, the welded joint can beproduced using much higher weld current. The weld nugget size made withresistance spot welding using direct current micro pulses is much largerthan the weld nugget size produced with conventional welding processes.As shown in FIG. 6, under the same conditions, conventional weldingtechniques may yield weld sizes no bigger than 5 mm, whereas the MPDCprocess produces weld sizes of 5 to 7 mm. Accordingly, the larger weldjoints achieved using direct current micro pulses translate into greaterweld joint strength than conventional welding methods. In one scenario,the fracture modes of peel samples for the MPDC methods were morefavorable because no interfacial fracture mode was observed.

Compared to conventional welding methods, the MPDC method may produce afiner microstructure in the weld nugget. FIGS. 7A and 7B areillustrations of the microstructures of the weld nuggets respectivelyformed by a conventional DC process and a MPDC process according to anembodiment of the invention. FIG. 7A illustrates the microstructure of aweld nugget produced by conventional DC welding. FIG. 7B illustrates aweld nugget produced by the MPDC process for 0.91 mm USIBOR 1500. Asshown in FIGS. 7A and 7B, the MPDC method produces a “clean” weld nuggetmicrostructure, which enhances the weld joint integrity. In someembodiments, the MPDC method produces weld nuggets without oxidizedAl/Si inclusions, which may yield stronger weld joints.

The MPDC method may breach the high resistive interfacial contact layerat joining surface with narrow weld current spikes, which makes themethod particularly suitable to join sheet metal materials withdifferent coatings (e.g. oxidized aluminum coating, hot dip galvanizedcoating) or oxidized steel surfaces.

Resistance spot welding with MPDC may have broad applications over awide variety of industries, including the automotive industry, appliancemanufacturing industry, aircraft manufacturing industry, agriculturemachinery manufacturing industry, and other manufacturing and/orfabricating industries. One advantage of methods and systems forresistance spot welding using direct current micro pulses are thebreadth of materials which can be welded. Some materials that cannot bewelded using conventional DC welding processes may be effectively weldedby using MPDC resistance spot welding.

The foregoing detailed description of the certain exemplary embodimentsof the invention has been provided for the purpose of explaining theprinciples of the invention and its practical application, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with various modifications as are suited to theparticular use contemplated. This description is not intended to beexhaustive or to limit the invention to the precise embodimentsdisclosed. Although only a few embodiments have been disclosed in detailabove, other embodiments are possible and the inventors intend these tobe encompassed within this specification and the scope of the appendedclaims. The specification describes specific examples to accomplish amore general goal that may be accomplished in another way. Modificationsand equivalents will be apparent to practitioners skilled in this artand are encompassed within the spirit and scope of the appended claimsand their appropriate equivalents. This disclosure is intended to beexemplary, and the claims are intended to cover any modification oralternative which might be predictable to a person having ordinary skillin the art.

Only those claims which use the words “means for” are to be interpretedunder 35 USC 112, sixth paragraph. Moreover, no limitations from thespecification are to be read into any claims, unless those limitationsare expressly included in the claims.

1. A method for resistance spot welding using direct current micropulses, comprising: forming a weld joint by applying a plurality ofdirect current micro pulses to at least two pieces of materials througha first electrode and a second electrode.
 2. The method of claim 1,further comprising: fixing the at least two pieces of materials togetherby a predetermined weld force.
 3. The method of claim 2, wherein thepredetermined weld force is between 1 kN and 10 kN.
 4. The method ofclaim 1, wherein a weld current off time separates each one of theplurality of direct current micro pulses.
 5. The method of claim 1,wherein a first one of the at least two pieces of material comprises afirst steel sheet and a second one of the at least two pieces ofmaterial comprises a second steel sheet.
 6. The method of claim 1,wherein the magnitude of each of the plurality of direct current micropulses is between 1 kA and 20 kA.
 7. The method of claim 1, wherein theduration of each of the plurality of direct current micro pulses isbetween 1 ms and 10 ms.
 8. The method of claim 4, wherein the durationof each of the plurality of weld current off times is between 1 ms and10 ms.
 9. The method of claim 1, wherein a magnitude of each of theplurality of direct current micro pulses is based at least in part on atleast one of a joint design of a weld joint joining the at least twopieces of material together, a gauge of at least one of the at least twopieces of materials, a coating of at least one of the at least twopieces of materials, a material chemical composition of at least one ofthe at least two pieces of materials, one or more mechanical propertiesof at least one of the at least two pieces of materials, a size of thepair of electrodes, or a magnitude of the predetermined weld force. 10.The method of claim 1, wherein the number of the plurality of directcurrent micro pulses is based at least in part on at least one of ajoint design of a weld joint joining the at least two pieces of materialtogether, a gauge of at least one of the at least two pieces ofmaterials, a coating of at least one of the at least two pieces ofmaterials, a material chemical composition of at least one of the atleast two pieces of materials, one or more mechanical properties of atleast one of the at least two pieces of materials, a size of the pair ofelectrodes, or a magnitude of the predetermined weld force.
 11. Themethod of claim 1, wherein a duration of each of said plurality ofdirect current micro pulses is based at least in part on at least one ofa joint design of a weld joint joining the at least two pieces ofmaterial together, a gauge of at least one of the at least two pieces ofmaterials, a coating of at least one of the at least two pieces ofmaterials, a material chemical composition of at least one of the atleast two pieces of materials, one or more mechanical properties of atleast one of the at least two pieces of materials, a size of the pair ofelectrodes, or a magnitude of the predetermined weld force.
 12. Themethod of claim 4, wherein the duration of each of said plurality ofweld current off times is based at least in part on at least one of ajoint design of a weld joint joining the at least two pieces of materialtogether, a gauge of at least one of the at least two pieces ofmaterials, a coating of at least one of the at least two pieces ofmaterials, a material chemical composition of at least one of the atleast two pieces of materials, one or more mechanical properties of atleast one of the at least two pieces of materials, a size of the pair ofelectrodes, or a magnitude of the predetermined weld force.
 13. Themethod of claim 1, wherein the weld joint comprises a finemicrostructure.
 14. A method for resistance spot welding using directcurrent micro pulses, comprising: fixing together a first piece ofmaterial and a second piece of material by a predetermined weld forcethrough a first electrode and a second electrode; and forming a weldjoint by applying a plurality of direct current micro pulses to thefirst piece of material and a second piece of material, wherein each ofthe plurality of direct current micro pulses are separated by aplurality of off times, each of the plurality of direct current micropulses have a duration of between 1 ms and 10 ms and a magnitude ofbetween 1 kA and 20 kA, and each of the plurality of off times have aduration of between 1 ms and 10 ms.
 15. A system for resistance spotwelding using direct current micro pulses, comprising: a first electrodeand a second electrode configured to form a weld joint joining at leasttwo pieces of materials together by applying a plurality of directcurrent micro pulses to the at least two pieces of materials.
 16. Thesystem of claim 15, further comprising a weld controller incommunication with the first electrode and a second electrode.
 17. Thesystem of claim 16, wherein the weld controller is configured to controlone or more weld parameters.
 18. The system of claim 17, wherein theweld parameters comprise the weld force, weld current magnitude, weldduration, total number of welds, and off-time duration.
 19. The systemof claim 17, wherein the weld controller controls the one or more weldparameters based at least in part on at least one of a joint design of aweld joint joining the at least two pieces of material together, a gaugeof at least one of the at least two pieces of materials, a coating of atleast one of the at least two pieces of materials, a material chemicalcomposition of at least one of the at least two pieces of materials, oneor more mechanical properties of at least one of the at least two piecesof materials, a size of the pair of electrodes, or a magnitude of theweld force.
 20. The system of claim 15, wherein the first electrode andthe second electrode are configured to fix the at least two pieces ofmaterials together by a predetermined weld force.
 21. The system ofclaim 20, wherein the predetermined weld force is between 1 kN and 10kN.
 22. The system of claim 15, wherein the magnitude of each of theplurality of direct current micro pulses is between 1 kA and 20 kA. 23.The system of claim 15, wherein the duration of each of the plurality ofdirect current micro pulses is between 1 ms and 10 ms.